There are many safety syringe designs available on the market. Most of these designs have as their major purpose the prevention of the spread of infectious diseases and in particular blood borne disease such as AIDS and Hepatitis C, from accidental needle stick to other persons after the needle has been used for a patient with such a disease. The simplest is some form of sheath or shield, which covers the needle either before and/or after use. Typical of these designs are U.S. Pat. No. 4,840,619 Hughes, U.S. Pat. No. 4,816,022 Poncy, U.S. Pat. No. 4,801,295 Spencer, U.S. Pat. No. 4,702,739 Milorad, U.S. Pat. No. 4,664,654 Strauss and U.S. Pat. No. 4,655,751 Harbaugh. These designs are useful in preventing needle stick when disposing of a used syringe but are cumbersome on the syringe, may not be properly deployed, are easy to remove and hence do not prevent improper reuse. It has therefore been proposed to make the needle retract wholly into the syringe body and to be trapped therein. Designs having such a retractable needle are U.S. Pat. No. 3,306,290 Weltman and U.S. Pat. No. 4,392,859 Dent.
These various devices all work well up to a degree but require that the operator manually withdraw the needle back into the barrel by pulling back the plunger rod. The operator may forget to do this, may be distracted before performing this operation, or may not have been instructed to do it. Also manual withdrawal requires that the operator change grip and to use both hands, which is a procedure during which the risk of needle stick to the operator is increased. To overcome these hazards it has been proposed to use spring loaded mechanisms which operate at the end of the injection stroke to push a slideable needle mounting into the barrel and to fully contain the needle therein. Designs that use internal springs to push the needle back into the barrel after use are U.S. Pat. No. 4,978,343 Dysarz et al., U.S. Pat. No. 4,955,868 Klein, and U.S. Pat. No. 5,049,133 Villen Pascual.
However the retraction mechanisms in these designs occupies a significant amount of the internal space of the syringe and hence the fluid volume is consequently significantly reduced. This means that the overall size of the syringe is significantly greater than a standard syringe of the same fluid capacity. Also these designs require full depression of the injection stroke to release the spring mechanism and in some instances the application of increased pressure at the bottom of the stroke to successfully release the spring and initiate retraction of the needle. Further in some of these designs the injection fluid comes into contact with the components of the retraction mechanism resulting in wastage due to fluid remaining in the spaces of the mechanism, and there is also the possibility of contamination of the injection fluid from the materials used in the mechanism.
The need has developed for a retractable type syringe which reliably and automatically withdraws the needle into the syringe body after use, and in which the retraction mechanism is compact such that the overall size of the syringe is not significantly greater than a standard syringe of the same fluid capacity, and in which the injection fluid does not come into contact with the components of the retraction mechanism.